This invention relates to the preparation of high molecular weight linear cyclic polyimides, which are thermoplastic substances. The term "thermoplastic" describes synthetic polymeric materials which are capable of being repeatedly softened when heated and hardened when cooled. Cyclic polyimides are known substances, having been described, inter alia, in U.S. Pat. No. 3,539,537 and in an article by Bower and Frost in J. Polymer Sci (A) 1, 3135 (1963). The methods described for preparing these polymers involve the preparation of prepolymers made from difficult to handle monomers (pyromellitic acid, dimethyl benzidine, etc.) in high boiling solvents (N-methyl pyrrolidone, tetramethylene sulfone, etc.). Generally, these methods yield cyclic polyimides of relatively low molecular weight. High molecular weight polymers can be obtained by this method only when using very pure monomers and by rigorous exclusion of moisture.
Another method of preparing a thermoplastic cyclic imide employs a solution copolymer of methacrylic acid and methacrylonitrile. In order to permit cyclization to the imide, the copolymer must have alternating units. There are various disadvantages to this currently used process. First, it is a solvent based process, thereby raising safety and environmental problems. The viscosity of the solution polymer is high and inconvenient to work with. As already noted, the unit sequence in the prepolymer is important since cyclization can take place only between a methacrylic acid unit and a methacrylonitrile unit. Moreover, if there are three or more consecutive nitrile units in the acid nitrile polymer chain, the condensation stops short, having one or more uncyclized nitrile units in the chain.
Accordingly, it is an object of this invention to provide an improved process for preparing linear cyclic polyimides of high molecular weight.
It is another object of this invention to provide a method for preparing polymeric cyclic imides which employs a water base emulsion system.
Yet another object is to prepare polymeric cyclic imides from a wide variety of prepolymers, particularly from copolymers in which there is no critical unit sequence.
And yet another object of this invention is to provide a wider variety of polymeric cyclic imide products than is currently available. Other objects and advantages of this invention will become apparent from the following discussion.